In an air-blowing working machine of portable type (hand-held type or shoulder type) such as a power blower and a sprayer for example, a centrifugal blower fan has been conventionally employed for generating a high-velocity air flow. This blower fan is generally constituted by an impeller including a predetermined number of blades which are radially arranged on a base; a fan casing having an air-intake port disposed to face a central portion of the base of the impeller and eccentrically housing the blower fan and defining a spiral scroll passageway between an inner wall thereof and an external circumference of the impeller, the spiral scroll passageway having an inlet portion which is constituted by a narrow nose portion and an outlet portion which is constituted by a wide discharge port; and a driving source for rotating the impeller such as an internal combustion engine and an electric motor (see for example, JP Laid-open Patent Publication (Kokai) No. 10-196596 (1998)).
This conventional blower fan however is accompanied with a problem of noise to be released to environments. This noise generates from the impingement of air against the narrow nose portion after the air has been most greatly compressed at region in the vicinity of the narrow nose portion. It is already known that the noise originating from this nose portion will be released directly from the air intake port to environments. In an attempt to minimize this noise, solutions such as modifying the configuration of blades or of impeller or to change the configuration of the air intake port (which is generally circular) as shown in JP Laid-open Patent Publication (Kokai) No. 1′-182499 (1999)) and JP Laid-open Patent Publication (Kokai) No. 7-91392 (1995)) have been proposed.
As described above, in order to overcome the problem of noise, the construction of the components of conventional blower fan needs to be modified to a great amount, resulting in the complication of the structure of blower fan and also in the deterioration in performance of blower fan (deterioration of air intake efficiency or air discharge capacity, etc.) for the sake of the reduction of noise.